Well, I can't comment on the age limit part, although Bud was already a year old when I got him, but I did notice before I trained him (and sometimes now, when he doesn't make it to the tub in time) that he has a tendency to go in one or two spots. If you find you can't tub train him, maybe you could put down a square of paper in his usual spot, then change to a container (with some water, maybe?) the same size as the paper.
Although I have to say, it was easier than that for me, because the place he tended to poop was his water bowl. (The other place is his basking spot. I'm not sure if that's laziness, or spite.
To train him, I started putting him in the tub of lukewarm, 4-5 inch deep water at 7:30 am and 7:30 pm, and gradually, he adjusted to go when he was in the water, since that's a natural poop inducer for them. He still has accidents though, and becomes completely untrained when I get back from trips longer than one day. 